The proposed research is designed to investigate the underlying cognitive causes for the language deficit in fragile X syndrome( FXS) and its relation to the expression of the fragile X gene. Individuals with FXS have consistently been shown to have deficits in language, especially in the pragmatic and discourse aspects of social communication. Despite the devastating effects this is likely to have on social functioning, the nature of these deficits has yet to be determined. The proposed research will examine language performance of adults with FXS on three underlying mechanisms of language processing: set shifting, working memory, and semantic inhibition. These will be examined in a sample of high functioning adults with FXS, compared to a clinical comparison group of adults with autism matched on VIQ and chronological age, and a control group of normally-developing children matched on verbal mental age. Based on previous research, it is predicted that adults with FXS will demonstrate significant impairments in set shifting, working memory, but will show relatively intact semantic inhibition. Moreover, the proposed research will determine the extent to which performance on experimental language measures relates to a measure of the pragmatic aspects of conversational speech that have been central in clinical descriptions of speech in FXS, but that are not adequately documented by conventional language measures. Beyond addressing these hypotheses, this study constitutes a comprehensive evaluation of key components of discourse and pragmatic ability in FXS, and thus provide important insights into the nature of the language deficit.